This invention relates to a method and an apparatus for controlling power stations wherein each generator is separately phase locked controlled. The control of power plants of the thermal, hydroelectric type, etc. is presently carried out by using as a main control quantity the input power in a generally analog manner, as well as other parameters that nevertheless require rather complex algorithms. Power adjustment must therefore be quick, feasible in a wide range of operating conditions, effective and reliable, considering that several energy conversions and high power values are involved. The relevant apparatuses are therefore rather expensive and attempt to reconcile opposite requirements.
The object of the present invention is a main control system, prevailingly electronic, that directly operates on the generator and by electric signals only for the main control in the short run. This way the secondary adjustment of the input power becomes easier and more effective and by means of an almost digital control.
The invention consists of a method and the relevant apparatus for use in a regulator of the locking type (phase locking, voltage locking, etc.) having features resembling those of a torque converter automatic change gear; i.e., having fixed the input power, if the load increases, the voltage decreases with an increase in the supplied current. This way, any change, e.g. a line voltage variation, provided that it is within predetermined limits, is automatically tracked and corrected very quickly independently of the input power magnitude. Although the input power can be subjected to changes, the reference frequency is maintained rigorously constant with very small phase variations.
In power plants wherein the size is relatively small with respect to the overall line power, the motor power can be fixed and adjusted for better efficiency or for maximum power without any specific feedback loop, or it can be variable (e.g., for hydroelectric stations) or even be moderately unstable. In all of these cases the input power control is limited by the alarm signaling since the instrumentation and the plant apparatuses are limited to the minimum requirement for the plant start and stop. In larger plants, the input power can be slowly proportioned, or programmed, the only aim being to approximately balance (match) the load while avoiding expensive and ineffective control systems. Thereby it is possible to prevent power excesses in standby conditions, to reduce the thermal stresses and to obtain a maximum of temperature and efficiency. Another advantage of the invention resides in eliminating the analog parameters from the feedback loop: the only reference parameters are time and a mechanical angle and both are digitized with high resolution and are substantially stable. The only analog signal is from the regulator output. The invention requires very few components, the performances of which may be only nominal.
In equilibrium conditions the input mechanical power is equal to the supplied electric power that is given by the product of the RMS voltage and the supplied current. The current value only depends upon the line impedance between the generator and the first network junction (i.e., upon the magnitude of the load and the corresponding difference of potential.) In the short run, disregarding the motor characteristics (that for small variations can be considered as supplying a constant torque), the kinetic rotational energy compensates for the difference between the motor power and the electric power by increasing or decreasing. Variations in kinetic energy (i,e., variation of the speed of rotation) can be detected as time difference (phase differences) between a reference pulse and a pulse generated by an encoder on the generator-motor shaft. These time differences allow, according to the invention, quick modification to the generator magnetic field by full electronic control, so as to prevent substantial faults.
If the motor slows down, the input power becomes smaller than the delivered power and, in order to balance the system, it is required to decrease the generator magnetic field, thus reducing the output voltage, the line potential difference and the output current. The field variation is proportional to the phase offset (time difference) between the reference and encoder pulses. To avoid false signals and too high response frequencies, it is preferred to provide means for gradually altering the field, such as a ramp voltage. The hysteresis allows for a certain phase tolerance. To obtain better time responses it is possible to resort to non-linear transfer functions which may be stored in a properly programmed memory, e.g., the integration of a few successive phase shift conditions, etc.